If a mobile station having only one wireless interface performs a handover in a wireless communication system, the mobile station searches a handover destination base station after disconnecting a connection with a base station being currently connected. Next, the mobile station selects the handover destination base station and connects to it from a base station found through the search. Therefore, with this procedure, the mobile station enters into an unconnected state with the base station at least while performing a connection procedure with the handover destination base station, which causes packet loss.
On the other hand, if a mobile station having two wireless interfaces performs the handover, while using one wireless interface to perform data transfer with the base station, the mobile station can use the other wireless interface to search a handover destination base station and connect to it. The mobile station can perform the handover after the connection with the handover destination base station has been completed, and the mobile station is connected to the base station even while performing the connection procedure with the handover destination base station. Therefore, a seamless handover can be realized without the unconnected state and the packet loss due to the unconnected state can be prevented. However, if the mobile station uses the two wireless interfaces, the mobile station needs to be equipped with means for merging received packets into one when the packets are received from the respective wireless interfaces, and means for sorting packets to be sent to the respective wireless interfaces.
However, when the handover is performed in the mobile station having the two wireless interfaces, different MAC addresses are to be fixedly assigned to each wireless interface, which causes the following problems.
The first problem is that since the different MAC addresses are fixedly assigned to each wireless interface, when a packet terminal connected to the mobile station performs data communication, the packet terminal must constantly identify and control multiple wireless interfaces during the data communication.
In order to solve this problem, it is conceivable to use a method of mounting a function on the packet terminal with which the multiple wireless interfaces can be dynamically recognized. However, it increases strains on the packet terminal and reduces a processing capability for the data communication.
Patent Document 1 (Japanese Patent No. 2790550) discloses a gateway device that has a high transfer processing capability and performs a packet routing among multiple local networks connected to each other. A network control unit of the gateway device determines a carrier route for a received packet, reassigns a MAC address of the packet according to the carrier route, and thereby realizes a packet transmission according to the reassigned MAC address. However, a process of reassigning the MAC address is not intended to work with the base station search process caused by a move of the mobile station. Therefore, when the packet terminal performs the data communication with the mobile station, the packet terminal cannot dynamically recognize the multiple wireless interfaces, and thus the first problem cannot be solved.
The second problem is depicted in FIG. 1. A wireless communication system, in which multiple base stations 201, 202 and 203 are connected to a layer 2 switch (L2SW (301)) that is a line concentrator having a switching function shown in FIG. 1, is taken as an example. A mobile station 101 is mounted with two wireless interfaces: namely, a wireless interface “a” including an antenna 11a and a wireless transmission unit 12a, and a wireless interface “b” including an antenna 11b and a wireless transmission unit 12b. It is assumed that the wireless interface a has been assigned with a MAC address xx:xx:xx:xx:xx:xx, and the wireless interface b has been assigned with a MAC address yy:yy:yy:yy:yy:yy.
If the mobile station 101 uses the wireless interface a to establish connection with the base station 201 and performs data transfer, the L2SW (301) updates information in a forwarding table FT for storing information on a forwarding process, with respect to the MAC address xx:xx:xx:xx:xx:xx. Next, when the mobile station 101 moves and performs the handover from the base station 201 to the base station 202, the mobile station 101 uses the wireless interface b to connect to the base station 202 and starts the data transfer. At this point, the L2SW (301) updates the information in the forwarding table FT, with respect to the MAC address yy:yy:yy:yy:yy:yy.
When using this method, if a frame having a destination MAC address of xx:xx:xx:xx:xx:xx arrives at the L2SW (301), the L2SW (301) refers to the forwarding table FT based on the MAC address xx:xx:xx:xx:xx:xx. Accordingly, even though the mobile station 101 is connected to the base station 202, the L2SW (301) transfers the data to the base station 201, thereby causing a problem of packet loss.
In order to solve this problem and to communicate with a mobile station having two wireless interfaces, a router may be installed on the base station side to deal with the mobile station's addresses and the handover will be performed. However, layer 3 connection control is required for managing a location of the mobile station. As a result, a strain required for the handover by the mobile station becomes larger compared with that caused by layer 2 control, and a data transfer processing capability deteriorates.
Patent Document 2 (Japanese Patent Application Laid-Open No. 2004-048334) discloses a data transfer apparatus and the like which enable uninterrupted data transfer when performing the handover in a mobile node (corresponding to the mobile station). For a switch installed at a higher level of multiple access points (corresponding to the base stations), the seamless handover is realized with a bi-cast transmission method, in which a packet bound for the mobile node is sent to both an access point being connected and an access point of a destination. However, the load required for the handover in the mobile node becomes large in association with previously sending the packet to both, and the second problem cannot be solved.